1. Field of the Invention
The present invention relates to an image reading device for reading an original image by relative movement of an original document and plural linear image sensors.
2. Related Background Art
There has already been proposed an image reading device in which plural linear image sensors are arranged along the main scanning direction thereof to constitute a long sensor. FIG. 1 shows an example of such an image reading device, composed of linear image sensors 1, 2, each of which has a linear array of plural photosensor elements. Three image sensors 1 are arranged on a line 1' on a substrate S, while two image sensors 2 are arranged on a different line 2'. The distance l between the lines 1' and 2' is generally selected as a multiple of the product of the scanning speed V in the sub-scanning direction of the image sensors or a direction substantially perpendicular to the direction of arrangement of said sensors, and the sampling interval T.
Such an image reading device makes possible high-speed image reading and compactization of apparatus utilizing such device.
FIG. 2 shows the mode of image reading with the image reading device shown in FIG. 1, and FIG. 3 shows the timing of such image reading. In FIG. 2, 4 indicates an image to be read, and oblong rectangular solid-lined areas 5, 6 indicate images corresponding to trains of pixels sampled at a point 11 shown in FIG. 3. Also, oblong rectangular broken-lined areas 7, 8 are images corresponding to trains of pixels sampled at a time 12 in FIG. 3. In the present example it is assumed that the distance l between the image sensors 1 and the image sensors 2 is equal to 5 times the product of the sub-scanning speed V and the sampling interval T. The image areas 6 sampled at the timing 11 shown in FIG. 3 can construct a continuous main scanning line on the image, in combination with the image areas 7 sampled at the time 12 which is 5 cycles later, and such construction is possible because the distance of the lines of image sensors is an integral multiple (5 times in this example) of the product of the sub-scanning speed V and the sampling interval T.
For the purpose of comparison, FIG. 4 shows a case in which the distance l of the lines of image sensors is 5.5 times of the product of the subs-scanning speed V and the sampling interval T. In the structure shown in FIG. 4, if the image sensors 1, 2 are operated with the same sampling pulses, the solid-lined image areas 6 are not in line with areas 7 sampled 5 cycles later (timing 12) nor with areas 9 sampled 6 cycles later (timing 13).
As explained above, such a staggered arrangement of linear image sensors is associated with a drawback of limitation in the scanning speed and the sampling interval depending on the distance of image sensors. Such drawback is not limited to the staggered arrangement as shown in FIG. 1 but appears in any arrangement in which plural linear image sensors are placed with a determined distance between them in the sub-scanning direction.